Vitor Pamplona isn’t a doctor. He’s not even an optician. He can’t write you a prescription for glasses, or sell you a pair. Still, he’s pretty sure he’s going to disrupt the $75 billion global eye-care market.

At EyeNetra, the startup he cofounded, goofy curiosities like plastic eyeballs line the shelves, and a 3-D printing machine whirs in the background. It’s printing out prototypes of a device that will attach to your smartphone and, in a minute or two, tell you what kind of eyeglasses you need.

The device, called the Netra-G, is based on some clever optics and software Pamplona came up with—a way to measure the refractive error of the eye using a smartphone screen and an inexpensive pair of plastic binoculars.